Infrared imaging optical systems are typically used to view and image light energy in the infrared optical spectrum of from about 2 to about 7 micrometers wavelength, and more particularly from about 3 to about 5 micrometers wavelength. The production of infrared light is associated in many cases with the evolution of heat by hot objects such as engines or by the human body, and for that reason infrared light is widely used to detect such hot objects. Infrared energy is transmitted through many conditions which block visible light, such as clouds of particulate matter or water vapor.
Missiles fired at an aircraft may be detected by the heat and corresponding infrared signatures produced by their engines, regardless of whether the missile is guided by an active or passive targeting system. Aircraft that are potentially targets for such infrared-guided missiles may carry infrared-warning devices that view the exterior world in search of heat signatures that are associated with the engines of the missiles. A warning signal is provided to the aircraft crew upon detection of such a signature.
In one type of infrared-warning device, fixed infrared warning sensors are positioned at locations on the target aircraft. Side-facing, top-facing, bottom-facing, forward-facing, and aft-facing sensors may be used. The sensors include an array of lenses that focus the external infrared energy onto a cryogenically cooled detector. The detector converts the incident infrared energy to electrical signals, which are analyzed for infrared signatures that may be associated with threats to the aircraft such as fired missiles.
The infrared-warning sensors may use an inverse-telephoto lens system, sometimes termed a “fisheye” lens, because it allows the field of view to be very large. Some associated distortion of the image is accepted, because the function of the infrared-warning sensor is not to precisely image the infrared signature, but instead to identify its presence and approximate location relative to the aircraft.
Inverse-telephoto optical systems are widely available for the visible spectrum. For infrared optical systems, however, many fewer types of inverse-telephoto optical systems are available, because the infrared detector must be cryogenically cooled. The lenses of the optical system are preferably not cooled, because a very large, high-capacity cryostat would be required and because the cooling from room temperature to the cryogenic operating temperature would alter the positions of the lenses due to thermal expansion. The inverse-telephoto lens array must therefore have an external pupil for the location of a cold shield that surrounds only the cryogenically cooled detector and not the lenses.
Only a very few inverse-telephoto optical systems, such as that described in U.S. Pat. No. 5,446,581, have an external pupil. These known external-pupil inverse-telephoto optical systems have a long ratio of their physical length to their focal length, typically on the order of 20:1 to 30:1. These very long lens systems result in a large weight and size for the infrared sensor, as well as require large-diameter lenses that are relatively expensive to manufacture. It is difficult to package the long infrared sensor for many locations where it would otherwise be desirable to position the sensor, such as at the top of the tail of the aircraft.
There is a need for an improved inverse-telephoto optical system for infrared applications which has an external pupil, and in which the wide field of view and fast optical speed are retained, but in which the optical system has a physical length that is much shorter than is now available. The present invention fulfills this need, and further provides related advantages.